1. Field of the Invention
This invention relates to an oxygen generator, having series connected cells, that can produce very high purity oxygen, with the cells based on the use of: a dense, solid oxide electrolyte; porous oxygen electrodes; porous air electrodes; and dense, cell-to-cell interconnections, which oxygen generator is particularly useful in an oxygen concentration apparatus.
2. Description of the Prior Art
Small oxygen concentrators are usually based upon pressure swing, nitrogen adsorption systems which concentrate the oxygen from ambient air to deliver approximately 95% O.sub.2 at about 3 liters of O.sub.2 /minute. U.S. Pat. No. 4,449,990 (Tedford) describes one such apparatus.
Although extremely reliable, these pressure swing adsorption units have certain disadvantages. They parasitically consume a major portion of the oxygen produced by back purging a standby tank of nitrogen adsorbent, in order to remove residual nitrogen, to rejuvenate the bed, prior to its next adsorption cycle. The units require timing and switching subsystems to activate a fixed time interval pressure swing in alternating between two nitrogen adsorption tanks. Also the nitrogen adsorbent can be rendered ineffective if exposed to excess humidity from the inlet air. In addition, purity is limited to approximately 95% O.sub.2.
Other oxygen-nitrogen separation devices are known, and taught by U.S. Pat. No. Re. 28,792 (Ruka et al.), where in one embodiment of the invention, single, tubular (ZrO.sub.2).sub.1-x (R.sub.y O.sub.z).sub.x solid electrolyte, where R is selected from Ca, Ba, Sr, Y, La, Sc, Yb and Sm, is coupled with a first electrode of porous lanthanum-nickel oxide or porous calcium-lanthanum-manganese oxide and a second electrode of nickel-platinum, or, alternatively, with a metal selected from Pt, Pd, Rh, Ir or their alloys as electrodes. A D.C. power source is used to transfer oxygen through the electrolyte.
Oxygen removal from air by dissociation of 02 into oxygen ions at a porous electrode-solid electrolyte interface is taught in U.S. Pat. No. 4,725,346 (Joshi). There, electric current flows with oxygen ions through a tubular, nonporous, thin electrolyte selected from solid zirconium oxide, hafnium oxide, cerium oxide or bismuth oxide. Exterior, porous electrodes, deposited on the electrolyte, are either silver, alloys of silver, or mixtures of silver and conductive ceramic oxides. Cell operation is in a sealed enclosure at over 500.degree. C. For medical oxygen generating devices, about 80% pure O.sub.2 is produced.
Solid oxide electrolyte cells and configurations are well known, and taught in U.S. Pat. Nos. 4,395,468 and 4,490,444 (Isenberg), but there, air and fuel are fed into the apparatus to generate electricity. This design involves a loosely sealed generator housing, with long tubular cells, each utilizing a single yttria-zirconia solid electrolyte, coupled with a single nickel-zirconia or cobalt-zirconia cermet fuel electrode, and a single air electrode selected from, for example, doped or undoped LaMnO.sub.3, CaMnO.sub.3, LaNiO.sub.3, LaCoO.sub.3, LaCrO.sub.3, and doped In.sub.2 O.sub.3, and also including an oxide doped (Ca, Sr, Mg) lanthanum chromite interconnection film. A different, stacked configuration, using primarily the same materials is taught in U.S. Pat. No. 4,728,584 (Isenberg). There, annular interconnection members separate cells, and the outer electrode of one cell is electronically and physically segmented from the inner electrode of an adjacent cell. End caps are also utilized.
Banded, series connected, stacked cell, solid oxide electrolyte fuel cell configurations are also well known, and taught, for example in U.S. Pat. No. 3,460,991 (White) and U.S. Pat. No. 3,525,646 (Tannenberger et al.). The former patent utilizes a stabilized ZrO.sub.2 solid electrolyte coupled with a first electrode selected from, for example, platinum, lithiated nickel oxide, and praseodymium cobaltate, and a second electrode selected from, for example, platinum, and nickel or silver in a mixture with conducting oxide material. Here, separate interconnections are not used between outer electrodes of one cell and inner electrodes of adjacent cells on the same tube. The latter patent utilizes a sealed conduit with an electrolyte of a ternary mixed oxide, such as, ZrO.sub.2 +CaO+MgO, a conductive wall material of nickel aluminide, coupled with a silver or nickel oxide/lithium oxide mixture as an outer electrode, and a supported inner electrode selected from, for example, iron, nickel, cobalt, copper, or their alloys. Here, electrolyte is disposed only on top of the base layer, not contacting the support.
The cells are shifted so that a conducting material disposed between electrolyte segments covers most of an insulating layer disposed completely between inner electrode segments.
White teaches a complex structure between cells that is not gas-impervious, since both overlapping electrodes are porous. Tannenberger et al. teach a complex arrangement of a minimum of six to seven cell components of dissimilar materials, and physical and chemical characteristics which makes the device extremely difficult to manufacture and operate in a reliable manner, especially with regard to gas tightness.
An alternate technology oxygen generator, overcoming state-of-the-art oxygen generator disadvantages in a system utilizing thin film technology, with the capability of producing high purity (99.9%) oxygen, and with the potential of being cost competitive, is needed. It is one of the objects of this invention to provide such a thin film oxygen generator.